Conventional subscriber systems, such as CATV systems, are typically arranged to provide a variety of services including video, data, audio, and telephony. The services available to the subscribers of a particular system are most often provided from a national network source and transmitted to the headend of the system over a satellite link. An integrated receiver-detector (IRD) located at the headend downlinks the services and additional headend equipment combines the satellite services with any local or terrestrial services into a broadband signal for transportation over the CATV distribution network.
There are many network architectures used in CATV systems for the delivery of the broadband signal to the subscribers. In the past, a common network architecture was a tree and branch structure having one or more coaxial cable trunks which covered the subscriber area. From each trunk, numerous coaxial cable feeders were then run to subscribers in smaller localities. Because optical fiber cable can carry more CATV services with less loss over greater distances than coaxial cable, many CATV systems are upgrading their coaxial cable plant with fiber optic cable. One popular modem architecture of a CATV system which uses optical fiber cable is termed a "star" configuration because it utilizes many optical fibers carrying the broadband signal from a central point to a plurality of local service areas or nodes. At each node, the broadband signal is converted back to a radio frequency (RF) broadband signal and distributed to subscribers via a series of coaxial cable feeders. A system that uses fiber optic cable and coaxial cable is called a hybrid fiber/coaxial (HF/C) system.
Generally, these CATV systems have been deployed as single point to multipoint broadcast type systems. Present-day CATV systems typically use a forward or downstream path over which the broadband video signal is broadcast, and a reverse or upstream path over which information may be returned to the headend. The upstream path is generally used for interactive data from the subscribers or for network control, billing, maintenance, and authorization functions. Interactive data can include data concerning a subscriber's instructions for ordering a product, feedback about on-screen information such as for interactive gaming or polling, a subscriber telephony or data signal directed to a person inside or outside the network, or the like. Thus the downstream broadband signal and the upstream signal for a CATV system are distinctly different from each other.
In the star architecture using fiber optic cable as the transmission medium to a series of nodes, one fiber can be utilized for the forward path and another fiber can be utilized for the reverse path. Current CATV architectures employ 1550-nm or 1310-nm wavelength optical signal beams to transport broadband video signals downstream using the forward path fiber. Interactive data and telephony traffic is transported upstream using 1310-nm wavelength optical signal beams over the reverse path fiber. Commercial high power optical amplifiers have made it possible to transport broadband video signals through forward path optical fibers which are deployed deeper, close to the subscriber's premises using 1550-nm wavelength lasers. Due to longer forward path fiber runs, reverse path optical fiber distances have proportionally increased. Therefore, upstream transmission of data, telephony, and video signals using 1310-nm wavelength lasers can be adversely affected because of additional attenuation over the longer reverse path fiber. An alternative for transporting upstream traffic is the use of 1550-nm wavelength laser sources where optical amplifiers can be employed to compensate for the excess loss in the reverse path fiber.
The use of separate downstream and upstream fibers also necessitates the use of additional optical amplifiers, which adds system expense. More systems are beginning to eliminate the necessity for a separate upstream fiber cable from each node by using optical multiplexing techniques to combine the downstream signal and upstream signal on a single fiber. However, because most optical amplifiers are unidirectional, there is still the additional expense of dual active components for the system.